The present invention relates generally to machines for making an aggregate for concrete and, more particularly, to a machine for making ground sand for concrete.
Rod mills for making crushed sand for concrete are well known. A rod mill includes a cylindrical crushing drum and a plurality of iron rods provided within the cylindrical drum. Rock fragments and water are supplied to the rotating crushing drum in which the iron rods exert impacts on the rock fragments to provide crushed sand.
In the above rod mill, the iron rods are lifted along the inside wall of the rotating drum and fall to crush the rock fragments. The hardness of the iron rods is much higher than that of the rock fragments so that the impacts upon the rock fragments are very high. The iron rods, however, do not sufficiently grind them to provide high quality sand.
Among river sand, mountain or pig sand, sea sand and land sand, the river sand is the hardest and has the most spherical shape and is the highest in quality as an aggregate. An experiment has shown that the particle shape percentage (unit volume mass divided by specific gravity in absolute dry condition) of a river sand sample prepared by washing with water according to Japan Industrial Standards (JIS) A 5004 is 57-59 percent while the particle shape percentage of the crushed sand obtained from the above rod mill is about 53 percent.
It is desirable that the particle size distribution curve falls between a pair of broken lines A and B in FIG. 13 (standard particle size range of a fine aggregate for concrete according to the Japanese Society of Civil Engineers). A test has shown that the particle size distribution curve of the crushed sands made by the rod mill does not always fall within the range.
In other words, the crushed sand made by the rod mill has flat and sharp particles with rough surfaces. Some of the particles even have a crack. These properties have adverse effects on the concrete characteristics such as workability and fluidity.
Hence, this inventor has proposed a machine for making ground sand for concrete in Japanese Patent Application No. 63-254539. This machine includes a cylindrical drum and a plurality of pebbles placed within the drum. Rock fragments (sand) and water are supplied to the rotating grinding drum through an inlet port on one end of the drum and ground sand is discharged through an outlet port on the other end of the drum. In this machine the rock fragments are ground by the pebbles and water with friction in a way similar to a river so that impurities are removed from the rock fragments, and sand having spherical particles with smooth surfaces without a crack are obtained in a short period of time.
The above machine, however, has the following disadvantages.
Rock fragments and water are supplied through the inlet port on one end of the drum and the ground sand and water is discharged through the outlet port on the opposite end. This outlet port is placed somewhere around the axis of the drum so that there is a very small centrifugal force acting on the ground sand in the vicinity of the outlet port, let alone a force acting along the axis. As a result, the fluidity of the ground sand near the outlet port is so low that the ground sand is not efficiently discharged through the outlet port.
In addition, water is supplied through the inlet port on only one end so that the water fails to sufficiently separate the ground sand from the working pebbles, impairing discharge of the ground sand.
Moreover, the inside surfaces of the end walls are so smooth that the rock fragments and working pebbles are slipping on the surface, failing to make any contribution to the grinding action within the drum.